Method of coagulating aerosols

ABSTRACT

A method of coagulating aerosols is disclosed which comprises setting up either planes or lines of particulate concentrations by means of one or more standing acoustic fields and then further concentrating and therefore coagulating the particulate material by propagating a sawtooth wave along the lines or planes of particulate concentration.

I United. States Patent 1 1 1111 3,771,286

Scott 5] Nov. 13, 1973 METHOD OF COAGULATING AEROSOLS 884,721 7/1953 Germany 55/277 [75] lnventor: David S. Scott, Oakville, Ontario, OTHER PUBLICATIONS Canada Acoustic Coagulation and Precipitation of Aerosols Assignee; Chub]; Industries Limited, East by P. Mednikov TH Consultants gramptomQmlal-iio, Canada Bureau, New York, 1965. V V Industrial and Engineering Chemistry Vol. 41, No. [22] 1972 11, Nov. 1949, Pages 2434-2442, entitled Agglomera- [21] Appl. No.: 223,514 tion of Smoke, Fog, or Dust Particles by Sonic Waves,

by Haw. St. Clair.

[52] U.S. Cl 55/15, 15/277, 15/292 Primary Examiner Dennis E Talbert Jr. [51] Int. Cl B01d 51/08 A IE N G bl 58 Field of Search 55/15, 277, 292; jf j g ifi 23/313, 314 0mey egrls [56] References Cited [.57] ABSTRACT UNITED STATES PATENTS A method of coagulating aerosols is disclosed which 2,215,484 9/1940 St. Clair 55/277 x Camprises Setting up either Planes P 2'949166 8/1960 Coleman et aim 55/277 late concentrations by means of one or more standing 3,337,759 8/1967 Daman 55/277 x acoustic fields and then further Concentrating and FOREIGN PATENTS OR APPLICATIONS therefore coagulatlng the particulate rnaterlal by propagatlng a sawtooth wave along the llnes or planes of 745,933 3/1956 Great Brltaln 23/314 particulate concentration 780,986 8/1957 Great Brltain.... 55/277 174,050 8/1952 Austria 55/277 14 Claims, 1 Drawing Figure METHOD OF COAGULATING AEROSOLS FIELD OF THE INVENTION BACKGROUND OF THE INVENTION In order to maintain an acceptable standard of air quality, it has been obvious for many years that it is necessary to develop new processes and to improve old processes for cleaning the gases emitted from industrial operations, transportation systems, power and heat generating plants and the like. Cleaning these gaseous emissions involves the separation and removal of gaseous and particulate constituents; the latter being commonly referred to as aerodisperse systems, aerosols, or particulate clouds. In the case of industrial operations, the economic advantages associated with the removal and recovery of valuable particulate material which would otherwise be lost to the atmosphere, is frequently a further motivation for effective gas cleaning. This is particularly true in the case of fine particulate matter and it is the primary purpose of the present invention to provide a method of separating such material from a carrier gas.

DISCUSSION OF THE PRIOR ART The prior art discloses a number of ways by means of which the solid particulate material in an aerosol may be coagulated or agglomerated in order to make the separation easier. One of the most striking characteristics of aerosols is their continuous and spontaneous coagulation. The particles, more or less independent of their material, stick together and agglomerate when they come in contact with one another. This process goes on continuously until the aerosol number density becomes so small that there are effectively no more collisions or until the particles become so large that they floculate out. Several commonly used air cleaning techniques make use of this property. These techniques enhance the probability that the particles will come in contact with one another and then collect the large agglomerated particles by relatively standard techniques. An electrostatic precipitator is an example of such a device. By charging the particles and applying an electric field, the particles are brought to a collecting plate where they agglomerate. At this point the macroparticles are easily collected, for example by vibrating or washing the collector plates. Simple filters are another example of a mechanical means by which the same general approach may be implemented.

It has been known for many years that the rate of agglomeration of aerosol particles can be increased by the application of strong sonic fields. This has not lead to widespread industrial application of devices making use of this phenomenon. The reason appears to be, at least in part, that few detailed studies of the acoustoaerosol processes which are involved when an intense acoustic field is impressed upon an aerosol have been undertaken. Instead, attempts have been made to simply apply the phenomenon directly to industrial scale apparatus with the result that the effectiveness of the apparatus has been unsatisfactory for widespread application.

However, the acoustic treatment of aerosols has certain distinct advantages for certain types of gas cleaning problems. For example, it has obvious advantages over electrostatic precipitation when the particulate matter has low resistivity, is excessively sticky, or explosive. Inaddition, when one or more conditions of high temperature, corrosiveness, stickiness, or fine particle size exist, sonic treatment can have significant advantages over filters or inertial separators.

DESCRIPTION OF THE DRAWINGS The single FIGURE of drawings illustrates schematically a form of apparatus with which the method of the present invention may be practised.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before discussing the apparatus by means of which the present invention may be practised and before discussing the method in detail, a brief discussion of the processes which take place within an aerosol when an intense acoustic field is impressed upon it may be useful.

Although the complete theory of particle motion and coagulation in an aerosol which is under the influence of a sonic field is not known and much of that which is speculated is controversial, it is believed that three types of mechanisms dominate. These are orthokinetic interactions, parakinetic interactions and acoustic drifts. In order to explain these mechanisms, a standing, or substantially resonant, planar acoustic field will be considered. In this sense, the volume in which the aerosol is being treated has antinodal and nodal acoustic regions; that is, regions in which there is substantial acoustic vibration and regions in which there is much less acoustic vibration, and these regions are essentially stationary with respect to the treatment chamber although the bulk aerosol may be continually flowing therethrough.

Orthokinetic interactions taken place when two or more aerosol particles which are fairly close together are located with their line of centers substantially parallel to the direction of vibration of the carrier gas (i.e. orthogonal to a wave front). For any given sound frequency, there is a range of particle sizes so large that inertial effects result in such particles remaining essentially stationary, while the carrier gas moves back and forth round them due to acoustic vibration. On the other hand, there is a range of particle size so small, that these particles will vibrate with virtually the same local displacement amplitude as the gas. Clearly, particles of different sizes vibrate with different amplitudes and phases. Hence, there is a differential motion established between particles of different sizes increasing the probability of their having a collision which would result in agglomeration. The degree of participation by an individual particle in the background gas acoustic motion is strongly dependent upon that particle's radius and the sound frequency. The movement decreases with increasing particle size for a given frequency and decreases with increasing frequency for a given particle size. Other factors also play a part in the amplitude and phase of particle motion such as, for example, particle material density.

Parakinetic interactions take place when two or more aerosol particles which are fairly close together are located with a component of their line of centers perpendicular (orthogonal) to the direction of vibration of the carrier gas (i.e. parallel to a wave front). Although the explanation of the mechanism by which these interactions take place is very controversial, it is generally agreed that the net effects are attractive; that is, two particles which are fairly close together and with their line of centers substantially perpendicular to the acoustic motion of the carrier gas tend to be attracted towards each other.

Acoustic drift is exhibited when a substantially resonant or standing acoustic field is present in an aerosol and results in particulate matter tending to drift towards the displacement antinodal regions. Under some circumstances the drift is towards the nodal regions. As is the case in parakinetic interactions, the mechanisms which cause this drift to occur are not fully understood. However, the fact is undisputed that an aerosol forms regions of increased particulate density (striations) in standing acoustic fields at locations which substantially correspond to antinodal or nodal planes.

In addition to the three mechanisms which have been described above, other phenomena occur in acoustoaerosol fields such as drifts due to acoustic turbulence and streaming. Moreover, those parameters which affect the coagulation rate of an aerosol in a quiescent gas, such as the presence of foreign vapors, particulate charge effects and the like obviously influence the coagulation rate in the presence of an acoustic field as well. These phenomena, however, are not important to an understanding of the method which constitutes the present invention.

In seeking ways by which the orthokinetic differential particle motion could be maximized, it was found that a series of plane propagating shocks (or sawtooth waves) have two advantages over the approximately sinusoidal standing wave configuration which has been used by most devices of the prior art. By a series of sawtooth waves is meant an acoustic field propagating in essentially one direction; that is, with no substantial reflection which would produce stationary nodal and antinodal regions.

The first advantage of such a field is that, resulting from the sudden change in the carrier gas velocity as each sawtooth wave passes a particular point in the aerosol, particles of all sizes have an initial differential motion with respect to the carrier gas as they pass through this velocity discontinuity. Hence, since different particle sizes have different relaxation times, all particles have periods during which they move differentially with respect to all particles larger or smaller than themselves. This is equivalent to having all the different standing wave frequencies necessary for all particle sizes to move with the gas or lag behind the gas.

The second advantage results from the fact that the maximum swept out" volume ofa vibrating particle in a standing wave is, in principle, that particles diameter multiplied by the local displacement-amplitude of the acoustic wave, since (unless the particle drifts) each vibration sweeps out the same volume. On the other hand, in a field of sawtooth waves propagating in one direction only, the passage of each wave front moves the particle backwards in the gas by a given amount and in the same direction, causing the particle to sweep out a new volume with each passing valve.

The method of the present invention contemplates the treatment of an aerosol in a coagulation chamber in which at least one standing acoustic field is set up with its wave front substantially parallel to the direction of aerosol flow through the chamber. The effect of this is to provide planes of particulate concentration at the nodal and antinodal planes. Sawtooth waves are then propagated to run parallel to these medal and antinodal planes thereby achieving a maximum agglomerating effect from both sonic fields.

Further, the method of the present invention contemplates the use of two mutually perpendicular (orthogonal) standing acoustic fields which, within the coagulation chamber, would establish nodal and antinodal lines rather than nodal and antionodal surfaces and the sawtooth wave may be then propagated in the third remaining mutually perpendicular (orthogonal) direction to run parallel to the nodal and antinodal lines.

The significant advantage of the method of the present invention is that the individual sonic fields are impressed upon the aerosol in such a way that each of the various mechanisms which take place within the aerosol may be maximized by the control of the sonic field which is responsible for the phenomenon.

The effect of parakinetic interactions in this sonic field configuration will now be discussed. Two mutually perpendicular drift producing fields will be considered for illustrative purposes in order that aerosol striation will occur about lines rather than about planes. In this circumstance it is seen that parakinetic attraction phenomena will occur when two particles lie in a plane substantially perpendicular to a striation axis. Since the particles along the striation axis are more or less surrounded by an asmuthially symmetric particle density they will, on average, experience no net force perpendicular (orthogonal) to the striation axis. On the other hand, particles located off the striation axis will see a negative radial gradient in number density (where the origin of the radius is the striation axis) and hence, on average, will experience a net force towards the striation axis. Thus, in addition to contributing directly to coagulation, parakinetic interactions systematically enhance the striation process.

In conventional acoustic precipitators, the aerosol flow is most often perpendicular to the phase planes and, in those few instances where it is not, the fluid dynamic channel is such that turbulence spreads the par ticulate matter throughout the entire volume of the device. As such, the particulate matter is continuously swept out of antinodal regions by the carrier gas which yields a fairly uniform dust loading throughout the entire volume of the device. In this circumstance, it is clear that the number of contacts between particles which result in coagulation varies in some inverse manner with dust loading. As a consequence, in practice, the dust loading should be noless than one grain per cubic foot in order that the aerosol responds satisfactorily to acoustic treatment in normal acoustic coagulation processes. On the other hand, in striated-shock coagulation of the present invention, the number density is systematically increased at the striations. Since, in addition, the orthokinetic motion continuously drives the particulate matter in one direction parallel to the striations, it is seen that, at least in principle, there is no lower limit to the grain or dust loading that can be effectively treated by the method of the present invention.

In order to prevent the aerosol striations from being broken up due to large scale turbulence or due to the overall gas flow, the flow through the coagulation chamber is normally kept essentially parallel to the striations and the channel configuration is normally such that, for the velocities involved, the flow does not yield large scale turbulence. For the velocities which would be encountered in most practical circumstances, this necessitates that the coagulation chamber have at least one cross-sectional dimension which is relatively small. For most industrial air cleaning requirements, this in turn requires that the complete facility be constructed of many, essentially similar, striated shock coagulation chambers operating in parallel.

As can be seen from the preceding discussion, the essence of the present invention is the special action of two or more separately identifiable acoustic fields at least two of which have essentially different functions in the process of the acoustic agglomeration of aerosols. By separating the fields which are primarily responsible for producing the orthokinetic motion and the striation-drift processes, each phenomenon may be independently maximized.

Referring now to the drawing, there will be described the construction of a single coagulation chamber which is suitable for practising the present invention.

The coagulation chamber-proper comprises an elongated chamber of rectangular cross-section denoted by reference numeral 10. As was mentioned earlier, in order that the flow remain essentially laminar through the coagulation chamber it is necessary that one dimension of the cross-sectional area be relatively small. Accordingly, a cross-sectional size in the range from 2 inches by 2 inches up to 8 inches by 6 feet appears to be appropriate. The length of the coagulation chamber will depend upon the velocity of the aerosol passing through it and the treatment time which is required. Treatment time is normally between 1 and 10 seconds and velocity is normally between 1 and 10 feet per second.

Fine particulate aerosol enters the coagulation chamber 10 from the inlet 11 at which point the flow is split into two passages 12 and 13 so that the flow may symmetrically enter the coagulation chamber around the sawtooth acoustic field generator 14 located within enclosure 15 lying axially spaced from the coagulation chamber 10. The sawtooth acoustic field generator may comprise, as illustrated schematically, an electrically driven generator 14 provided with a horn 16 directed so as to propagate the sawtooth wave in a direction parallel to the flow of aerosol through the coagulation chamber. There are a number of other ways in which the sawtooth wave may be generated such as pulsating combustion sources, spark-gap sources, sirens and the like. In those circumstances where the wave form is not properly developed into a sawtooth configuration at its source, a region may be required in which there is essentially clean gas through which the wave may pass and thereby steepen into the sawtooth form which is required. This takes place in the shock development region 17 which, depending upon the other dimensions of the apparatus may be as long as 20 feet in axial extent.

Located on at least one and preferably two sides of the coagulation chamber 10 are the striation field generators schematically illustrated at 18. if a single standing acoustic field is to be set up within the coagulation chamber 10 this may be produced by generating a standing field about the location of one wall or of about two opposing walls. Whether one or two walls are used will depend upon aerosol characteristics, frequencies and pressure levels required and the size of the channel. The standing acoustic field may be produced by any one of a number of means including membrane oscillators, horns, sirens and the like.

Where two mutually perpendicular standing acoustic fields are to be set up within the coagulation chamber 10 the field generators will be located on at least two mutually perpendicular walls of the coagulation chamber 10 and, depending upon the factors enumerated above, the striation field generators may be located on all four walls of the coagulation chamber.

Below the coagulation chamber and downstream from the treatment zone a baffle arrangement is normally provided to prevent the reflection of the sawtooth wave at the end of the coagulation chamber remote from the end at which the sawtooth wave is propagated. This baffle arrangement is schematically illustrated by central bafflel9 and side baffles 20 which are extensions of the side walls of the chamber 10. Subsequent to passing the baffle zone where the sawtooth wave is diffused at the downstream end of the coagulation chamber 10 the coagulated aerosol comprising coarser particulate material passes out of exit 21 from where it may be passed to a settling chamber or to some secondary treatment.

The sawtooth field which is propagated axially of the coagulation chamber has been found to be satisfactory at a frequency of between 50 and 500 cycles per second with sound pressure levels between and 180 db.

The striation field, the standing acoustic field within the'coagulation chamber 10 has been found to be satisfactory at a frequency of between 500 and 5,000 cycles per second at sound pressure levels between 140 and db.

As the sawtooth wave propagates axially through the coagulation chamber it will normally dissipate and decrease in intensity. This intensity can be modified as it passes through the coagulation chamber by gradually varying the cross-sectional size of the coagulation chamber in the direction of aerosol flow therethrough. If the aerosolchamber 10 is gradually tapered inwardly from its upper end to its lower end as seen in the drawing the decrease in intensity of the sawtooth wave will be diminished as it propagates through the aerosol and the intensity will remain higher than would be'the case if the cross-sectional size of the coagulation chambe 10 were to remain uniform.

Conversely, the coagulation chamber may be widened in the downward direction in order to further weaken the strength of the shocks in order that particulate matter which has agglomerated will not be rebroken up by sawtooth waves of too high an intensity.

It is to be understood that the apparatus schematically disclosed herein is intended only to be illustrative of apparatus which is suitable for use in the practice of the method disclosed and claimed herein. Further, it is repeated that for most industrial applications, it will be necessary to arrange a plurality of coagulation chambers 10 in parallel so as to provide the flow capacity necessary to handle the gases to be cleaned.

I claim:

1. A method of coagulating aerosols comprising the steps of:

a. passing the aerosol through a coagulation chamber;

b. setting up at least one substantially standing acoustic field in the aerosol within the coagulation chamber with its wave front substantially parallel to the direction of aerosol flow through the chamber, and

c. propagating a sawtooth wave through the aerosol along and in the same sense as the direction of aerosol flow through the coagulation chamber.

2. A method as claimed in claim 1, wherein the sawtooth wave is diffused at the end of the chamber remote from the end from which it is propagated.

3. A method of coagulating aerosols as claimed in claim 1 wherein the reduction in intensity ofthe sawtooth wave as it traverses the length of the coagulation chamber is diminished by means of a gradual reduction in cross-sectional area of the coagulation chamber in the direction of aerosol flow therethrough.

4. A method of coagulating aerosols as claimed in claim 1 wherein the reduction in intensity of the sawtooth wave as it traverses the coagulation chamber is increased by a gradual increase in the cross-sectional area of the coagulation chamber in the direction of aerosol flow therethrough.

5. A method of coagulating aerosols as claimed in claim 1 wherein the aerosol passing axially through the elongated coagulation chamber is maintained under substantially laminar flow conditions.

6. A method of coagulating aerosols as claimed in claim 1 wherein the aerosol is passed axially through the elongated coagulation chamber at a rate of flow of between 1 to 10 feet per second.

7. A method of coagulating aerosols as claimed in claim 1 wherein at least one standing acoustic field has a frequency of between 500 and 5,000 Hz and a pressure level of from M to 165 db.

8. A method of coagulating aerosols as claimed in claim 1 wherein the sawtooth wave is of a frequency of between 50 and 500 Hz and has a pressure level of from 155 to 180 db.

9. A method of coagulating aerosols comprising the steps of:

a. passing the aerosol axially through an elongated coagulation chamber;

b. setting up a first substantially standing acoustic field in the aerosol in the coagulation chamber with its wave front substantially parallel to the axis of the chamber;

c. setting up a second substantially standing acoustic field in the aerosol in the coagulation chamber with its wave front substantially parallel to the axis of the chamber and substantially normal to the plane of the wave front of the first standing acoustic field, and

d. propagating a sawtooth wave through the aerosol along and in the same sense as the direction of aerosol flow through the coagulation chamber.

10. A method of coagulating aerosols as claimed in claim 9 wherein the reduction in intensity of the sawtooth wave as it traverses the length of the coagulation chamber is diminished by means of a gradual reduction in cross-sectional area of the coagulation chamber in the direction of aerosol flow therethrough.

11. A method of coagulating aerosols as claimed in claim 9 wherein the reduction in intensity of the sawtooth wave as it traverses the coagulationchamber is increased by a gradual increase in the cross-sectional area of the coagulation chamber in the direction of aerosol flow therethrough.

12. A method of coagulating aerosols as claimed in claim 9 wherein the first and second standing acoustic fields are of a frequency between 500 and 5,000 Hz and have a pressure level of from to db.

13. A method of coagulating aerosols as claimed in claim 9 wherein the sawtooth wave is of a frequency of from 50 to 500 Hz and has a pressure level of between 155 and db.

14. A method of coagulating aerosols comprising the steps of:

a. passing the aerosol axially through a plurality of elongated coagulation chambers grouped in parallel;

b. setting up at least one standing acoustic field within the aerosol in each coagulation chamber with the wave front of the acoustic field being substantially parallel to the directions of aerosol flow through the chambers, and

c. propagating a sawtooth wave through the aerosol along and in the same sense as the direction of aerosol flow through each coagulation chamber. 

2. A method as claimed in claim 1, wherein the sawtooth wave is diffused at the end of the chamber remote from the end from which it is propagated.
 3. A method of coagulating aerosols as claimed in claim 1 wherein the reduction in intensity of the sawtooth wave as it traverses the length of the coagulation chamber is diminished by means of a gradual reduction in cross-sectional area of the coagulation chamber in the direction of aerosol flow therethrough.
 4. A method of coagulating aerosols as claimed in claim 1 wherein the reduction in intensity of the sawtooth wave as it traverses the coagulation chamber is increased by a gradual increase in the cross-sectional area of the coagulation chamber in the direction of aerosol flow therethrough.
 5. A method of coagulating aerosols as claimed in claim 1 wherein the aerosol passing axially through the elongated coagulation chamber is maintained under substantially laminar flow conditions.
 6. A method of coagulating aerosols as claimed in claim 1 wherein the aerosol is passed axially through the elongated coagulation chamber at a rate of flow of between 1 to 10 feet per second.
 7. A method of coagulating aerosols as claimed in claim 1 wherein at least one standing acoustic field has a frequency of between 500 and 5,000 Hz and a pressure level of from 140 to 165 db.
 8. A method of coagulating aerosols as claimed in claim 1 wherein the sawtooth wave is of a frequency of between 50 and 500 Hz and has a pressure level of from 155 to 180 db.
 9. A method of coagulating aerosols comprising the steps of: a. passing the aerosol axially through an elongated coagulation chamber; b. setting up a first substantially standing acoustic field in the aerosol in the cOagulation chamber with its wave front substantially parallel to the axis of the chamber; c. setting up a second substantially standing acoustic field in the aerosol in the coagulation chamber with its wave front substantially parallel to the axis of the chamber and substantially normal to the plane of the wave front of the first standing acoustic field, and d. propagating a sawtooth wave through the aerosol along and in the same sense as the direction of aerosol flow through the coagulation chamber.
 10. A method of coagulating aerosols as claimed in claim 9 wherein the reduction in intensity of the sawtooth wave as it traverses the length of the coagulation chamber is diminished by means of a gradual reduction in cross-sectional area of the coagulation chamber in the direction of aerosol flow therethrough.
 11. A method of coagulating aerosols as claimed in claim 9 wherein the reduction in intensity of the sawtooth wave as it traverses the coagulation chamber is increased by a gradual increase in the cross-sectional area of the coagulation chamber in the direction of aerosol flow therethrough.
 12. A method of coagulating aerosols as claimed in claim 9 wherein the first and second standing acoustic fields are of a frequency between 500 and 5,000 Hz and have a pressure level of from 140 to 165 db.
 13. A method of coagulating aerosols as claimed in claim 9 wherein the sawtooth wave is of a frequency of from 50 to 500 Hz and has a pressure level of between 155 and 180 db.
 14. A method of coagulating aerosols comprising the steps of: a. passing the aerosol axially through a plurality of elongated coagulation chambers grouped in parallel; b. setting up at least one standing acoustic field within the aerosol in each coagulation chamber with the wave front of the acoustic field being substantially parallel to the directions of aerosol flow through the chambers, and c. propagating a sawtooth wave through the aerosol along and in the same sense as the direction of aerosol flow through each coagulation chamber. 